The present invention relates generally to fuel-fired forced air heating furnace apparatus, and more particularly relates to a condensing type furnace having improved construction features and operating characteristics.
For many years, conventional fuel-fired forced air heating furnaces were (and still are) provided with a combustor and a single heat exchanger to transfer combustion product heat to the supply air being delivered to the conditioned space served by the furnace. The single heat exchanger, typically of the flat "clamshell" type, normally provides the furnace with an overall heating efficiency of approximately 70-75%, and generally removes a sensible heat portion of the total combustion product heat available. When heating fuel was relatively inexpensive, this was considered to be an acceptable heating efficiency even though a substantial portion of the available heat was sent up the flue in the form of hot exhaust gas.
With the dramatic escalation in heating fuel costs, however, vigorous design efforts were initiated to improve furnace heating efficiency. One method proposed was to additionally capture a portion of the latent heat of condensation of the combustion gases by adding to the system a secondary heat exchanger connected downstream of and in series with the primary heat exchanger. The secondary heat exchanger functions to partially condense the combustion gases discharged from the primary heat exchanger, thereby capturing a portion of the available combustion gas latent heat in addition to the sensible heat captured by the primary heat exchanger and transferred to the heating air. Furnaces utilizing this primary/secondary heat exchanger format are commonly referred to as "recuperative" or "condensing" furnaces.
While condensing furnaces of conventional construction and operation typically achieve a significant increase in overall heating efficiency compared to single heat exchanger furnaces, they typically have presented new problems and limitations which to a large extend have tended to offset their heating efficiency improvements For example, many conventional condensing furnaces are designed around "clamshell" type primary heat exchangers, a design which limits both the cost effectiveness and the feature set of the resulting condensing furnace.
Specifically, the cost effectiveness of conventional clamshell-designed condensing furnaces is typically limited by, among other things, the total number of parts required to fabricate the furnace, non-automated assembly techniques required to fabricate the heat exchanger portion of the furnace, the enlarged size of the resulting furnace, and the degree of obtainable thermal cooperation between the clamshell primary heat exchanger and its associated secondary heat exchanger.
Additionally, the feature set of conventional condensing furnaces utilizing clamshell type primary heat exchangers is typically limited by, among other things, the inability to obtain a totally sealed combustion system, a non-integral condensate drainage system, non-integral combustion air supply and venting connections, and the necessity of utilizing high excess air/lanced port burners.
In view of the foregoing, it is accordingly an object of the present invention to provide a condensing furnace which, compared to conventional condensing furnaces, has an improved construction and improved operating characteristics.